Enzyme Inhibition Calculator Formula
Understand the math behind the enzyme inhibition calculator. Each variable explained with a worked example.
Formulas Used
Apparent Km (Competitive)
km_app_comp = km * alphaVelocity (Competitive Inhibition)
v_competitive = vmax * substrate / (km * alpha + substrate)Velocity (Noncompetitive Inhibition)
v_noncompetitive = vmax / alpha * substrate / (km + substrate)Variables
| Variable | Description | Default |
|---|---|---|
vmax | Vmax (uninhibited)(umol/min) | 100 |
km | Km (uninhibited)(uM) | 50 |
inhibitor | Inhibitor Concentration [I](uM) | 20 |
ki | Inhibition Constant (Ki)(uM) | 10 |
substrate | Substrate Concentration [S](uM) | 50 |
alpha | Derived value= 1 + inhibitor / ki | calculated |
How It Works
Enzyme Inhibition Types
Competitive Inhibition
Inhibitor binds to the active site, increasing apparent Km while Vmax remains unchanged.v = Vmax × [S] / (alpha × Km + [S]), where alpha = 1 + [I]/Ki
Noncompetitive Inhibition
Inhibitor binds away from the active site, decreasing apparent Vmax while Km remains unchanged.v = (Vmax/alpha) × [S] / (Km + [S])
Competitive inhibition can be overcome by high substrate concentration; noncompetitive cannot.
Worked Example
Vmax=100, Km=50 uM, [I]=20 uM, Ki=10 uM, [S]=50 uM.
- 01alpha = 1 + 20/10 = 3.0
- 02Competitive: v = 100 × 50 / (50 × 3 + 50) = 5000 / 200 = 25.0 umol/min
- 03Noncompetitive: v = (100/3) × 50 / (50 + 50) = 33.33 × 0.5 = 16.67 umol/min
Frequently Asked Questions
How do I distinguish competitive from noncompetitive?
On a Lineweaver-Burk plot: competitive inhibition changes the slope but not the y-intercept (lines converge on y-axis). Noncompetitive changes the y-intercept but not the x-intercept (lines converge on x-axis).
What is uncompetitive inhibition?
Uncompetitive inhibitors bind only to the enzyme-substrate complex, decreasing both apparent Vmax and Km by the same factor. On LB plots, this gives parallel lines.
How is Ki determined experimentally?
Measure kinetics at several inhibitor concentrations. Plot apparent Km (competitive) or apparent 1/Vmax (noncompetitive) vs [I]. The x-intercept of this secondary plot gives -Ki.
Ready to run the numbers?
Open Enzyme Inhibition Calculator